The goal of this research is to account for the rapid mental computations performed on information encoded from a visual display. The empirical work addresses five main content areas: (1) mental transformation of spatial information, such as transformation of orientation or size, (2) mental comparison of visual multidimensional stimuli, (3) picture encoding and recognition, (4) comparing linguistic descriptions to visual scenes, and (5) construction and modification of information processing strategies by adults and children in simple visual tasks. The methodology is to track subjects' eye fixations as they perform the task. The fixation behavior, the locus, duration, and sequence of fixations, is used to make inferences about the ongoing cognitive processes. The use of eye fixations is a convenient means to study cognitive operations; the focus of the research is not on sensory or perceptual processes. The resulting processing models for individual tasks and the meta-theory to integrate the models will be expressed as production systems, a formalism particularly suited to modelling individual mental operations. The theory will provide a list of mental operations, their characteristics and organization. The content area, methodology, and theoretical framework are all mutually compatible. They should deliver testable models and a reliable data base for the understanding of visual information processing.